1. Field of the Invention
This invention relates to methods for removing petroleum coke deposits, and specifically, furfural coke. Coke can arise from a high temperature condensation of hydrocarbyl species, i.e. materials containing primarily hydrogen and carbon with minor amounts of heteroatoms such as nitrogen, oxygen, or sulfur. Furfural coke, for purposes of this specification, means hydrogen deficient hydrocarbyl species resulting from decomposition, autoxidation, or polymerization of furfural.
2. Prior Art
On page 181 of Hydrocarbon Processing published by Gulf Publishing Company, May, 1978, an article entitled "Problems in Furfural Extraction" discusses problems of coking in lube oil plants. Coke deposits apparently arise due to decomposition, autoxidation or polymerization of furfural. Such furfural decomposition is reported to be inhibited with sodium bicarbonate or tertiary amines which apparently neutralize acids formed during decomposition. An example of a lube oil plant utilizing furfural extractions is reported in the September, 1982, issue of Hydrocarbon Processing on page 184. However, regardless of the steps to inhibit coke from furfural decomposition which are presently available, there eventually results such an accumulation of such coke that its removal from the system is required.
Current methods for removing coke have all proven to be too troublesome and time consuming. Scraping with water jets fails because furfural coke has a very high crush strength. Simply scraping or chipping is unsatisfactory because when coke is on the shell side of exchangers, only the outer rows of tubes are accessible. Other methods for cleaning out furfural coke deposits include letting the coke weather in the open for several months, then cleaning with a jet of high pressure water (Texaco's method). This works if the metallic surface is aluminum; it is not known if it will work if the surface is carbon or stainless steel.
Furfural coke deposits are particularly difficult to remove because furfural coke is much harder and clings to metal surfaces more than conventional petroleum coke.
In general, there is no rapid and economically efficient method for removing furfural coke deposits known to the prior art. Conventional methods have been found to be very difficult and inefficient in removing coke deposits. Examples of traditional methods for removing coke deposits are chipping, water jet, steam cutting, and sawing.
Though furfural, as any coke, can be burned, simply burning coke out of a metal heat exchanger is undesirable, because high temperatures from such burning can lead to warping and the introduction of strains and stresses within the metal. Even the physical properties of specially prepared metals such as stainless steel or chrome in the presence of carbon at high temperatures, such as above 800.degree. F. (427.degree. C.), often deteriorate so that they are no longer as corrosion resistant as they would otherwise be had they been properly treated.
The ideal situation is to have a very quick, easy and cost effective method to remove furfural coke deposits. Specifically, it would be desirable to have a method for quickly removing furfural coke from exchangers with blockage ranging from 1/8" to completely plugged. This usually represents roughly two to four years of properly buffered operation.
R. P. Van Driesen in U.S. Pat. No. 3.420,711 ("711") discloses a process for defouling process equipment. Specifically, combustible carbonaceous deposits resulting, for example, from hydrocracking, thermal cracking, polymerization or like high pressure processes are removed by oxidation under super atmospheric pressure which is sufficient to maintain at least a portion of water in a liquid phase. The presence of a water phase is critical to the process of '711 in order to absorb heat given off during combustion to avoid an excessive temperature increase. The process conditions of '711 are: a temperature not in excess of 700.degree. F.; a pressure maintained in the range from about 300 p.s.i.g. (pounds per square inch gauge) to about 5000 p.s.i.g.; a partial pressure of molecular oxygen of at least about 10 p.s.i. (pounds per square inch) preferably 100 to 500 p.s.i. Of critical importance to the defouling operation of Van Driesen is maintaining metal surfaces in contact with a continuous liquid phase of water.
Accordingly, an object of this invention is to provide a method for removing furfural coke deposits, for example, from heat exchangers in a rapid and efficient manner without deteriorating any metallurgical properties of surfaces from which such coke is removed.